> How do you prove anything, after the key material is compromised?

It’s a blockchain, so the simplest would be chain of custody until the chain points undeniably at you. This is not a pure cryptographic device, some social intervention might be needed here.

Even if Q-day means there is a way to deterministically retrieve any private key from a public key (is that what it means? or is the blast radius of q-day contained? This is a bit above my level of cryptography), I’m sure we could come up with something to minimize the damage. In the worst case, it might involve a claim process with an authority or consensus mechanism to prove who the rightful owner of the funds is and revert the unauthorized transactions on the new chain.

Yes, this is not ideal! But if the wallet conversion requires active participation, preemptive measures are also not ideal.

One thing that is not addressed: say this quantum attack happens tomorrow and everyone agrees it was an attack, what would prevent the community (miners, node operators, and users) to hard fork the chain at a snapshot before the attack, patch the protocol, and call that Bitcoin? There would be loss of value of course, but it is not unrecoverable.

It’s worth remembering that Ethereum forked for much less (not even a bug in the protocol, but a bug in a private application running on the protocol) and nobody seems too upset about it a decade later.